1. Field of the Invention
The present invention relates to an internal combustion engine including a supercharger provided on an intake passage, which is electric-motor-assisted by an electric motor.
2. Description of the Related Art
For improving fuel economy of automobiles, a technology of providing a supercharger, which is driven by an electric motor, on an intake passage to increase an output of an engine (internal combustion engine) is known. The electric motor is provided on a shaft of a turbine and a compressor and has the following characteristics. The electric motor rotates in the case where a desired supercharging pressure is not obtained because of the shortage of an exhaust gas when supercharging is required or in the case where the supercharger rotates at a low speed. In the case where the turbine is rotated by the exhaust gas to successfully obtain the desired supercharging pressure, the electric motor is rotated by the exhaust gas.
On the other hand, when the supercharging is required, a mechanism including a turbine on an exhaust passage converts an exhaust gas energy into a rotational energy by the turbine to drive a compressor coupled to the turbine through an intermediation of a rotary shaft, thereby performing supercharging work. However, when the supercharging is not required or high load is applied, the turbine acts as an exhaust resistance. As a result, fuel-economy performance is degraded.
Moreover, a turbocharger generally includes a valve called a west gate valve for causing the exhaust gas to bypass the turbine to prevent the load from being too high due to the exhaust gas when the engine is operated with high load. Therefore, the turbocharger has a flow path for causing the exhaust gas to bypass the turbine when the load is high. In this manner, the exhaust gas is caused to bypass a turbine wheel to prevent the exhaust resistance from increasing until the supercharging pressure rises (for example, see JP 2005-171896 A).
In this case, there is no exhaust resistance because the exhaust bypass path is opened to cause the exhaust flow to bypass the turbine wheel until the supercharging pressure rises. However, it is necessary to increase the supercharging pressure by the rotation of the electric motor in the meantime, resulting in consumption of a significant amount of power. Therefore, there is a problem that the fuel-economy performance is likely to be consequently degraded.